1. Field of the Invention
The present invention relates to an improved thermal conditioning apparatus and methods of using the same. More particularly, the present invention relates to improved thermal conditioning plate and method for use in controlling the temperature in the placement and curing of photoresist on a semiconductor substrate wafer.
2. Description of the Invention Background
Integrated circuits are typically constructed by depositing a series of individual layers of predetermined materials on a wafer shaped semiconductor substrate, or "wafer". The individual layers of the integrated circuit are, in turn, produced by a series of manufacturing steps. For example, in forming an individual circuit layer on a wafer containing a previously formed circuit layer, an oxide, such as silicon dioxide, is deposited over the previously formed circuit layer to provide an insulating layer for the circuit. A pattern for the next circuit layer is then formed on the wafer using a radiation alterable material, known as photoresist. Photoresist materials are generally composed of a mixture of organic resins, sensitizers and solvents. Sensitizers are compounds, such as diazonaphtaquinones, that undergo a chemical change upon exposure to radiant energy, such as visible and ultraviolet light, resulting in an irradiated material having differing salvation characteristics with respect to various solvents than the nonirradiated material. Resins are used to provide mechanical strength to the photoresist and the solvents serve to lower the viscosity of the photoresist so that it can be uniformly applied to the surface of the wafers. After a photoresist layer is applied to the wafer surface, the solvents are evaporated and the photoresist layer is hardened, usually by heat treating the wafer. The photoresist layer is then selectively irradiated by placing a radiation opaque mask containing a transparent portion defining the pattern for the next circuit layer over the photoresist layer and then exposing the photoresist layer to radiation. The photoresist layer is then exposed to a chemical, known as developer, in which either the irradiated or the nonirradiated photoresist is soluble and the photoresist is removed in the pattern defined by the mask, selectively exposing portions of the underlying insulating layer. The exposed portions of the insulating layer are then selectively removed using an etchant to expose corresponding sections of the underlying circuit layer. The photoresist must be resistant to the etchant, so as to limit the attack of the etchant to only the exposed portions of the insulating layer. Alternatively, the exposed underlying layer(s) may be implanted with ions which do not penetrate the photoresist layer thereby selectively penetrating only those portions of the underlying layer not covered by the photoresist. The remaining photoresist is then stripped using either a solvent, or a strong oxidizer in the form of a liquid or a gas in the plasma state. The next layer is then deposited and the process is repeated until fabrication of the semiconductor device is complete.
Photoresist and developer materials are typically applied to the wafer using a spin coating technique in which the photoresist is sprayed on the surface of the wafer as the wafer is spun on a rotating chuck. The spinning of the wafer distributes the photoresist over the surface of the material and exerts a shearing force that separates the excess photoresist from the wafer thereby providing for a thin layer of photoresist on the surface of the wafer. Following the spin coating of the wafer, the coating is heated, or soft baked, to remove the volatile solvent components, thereby hardening the photoresist.
The properties of the photoresist, and, therefore, the suitability of the photoresist for use in the subsequent processing steps, are largely dependent upon the ability to uniformly harden the photoresist. The heating of the photoresist can be performed either by convection, infrared heating or through the use of a hot plate. While convection and infrared heating can be performed in bulk, the use of a hot plate to individually bake the wafer on a heating surface has become the preferred method. This is because the hot plate method provides for rapid heating of the wafer and the heating occurs from the wafer-photoresist interface toward the surface of the photoresist, which tends to drive off gas pockets present in the photoresist and also prevents the formation of a surface crust or the photoresist. In order for the hot plate soft baking technique to be cost effective in comparison with the batch techniques, an automated wafer handling system must be used to maximize the throughput of the wafers. In addition, cooling assemblies are often employed to reduce the cooling time for the wafer so as to enhance the overall throughput of the system. As such, the heating and cooling system are directly tied into the automated wafer handling system.
A problem that arises with the prior art integrated spin coating systems is that when the heating or cooling assemblies must be repaired or replaced, extensive and costly amounts of downtime occur because of the integration of the system. The costs are especially significant in a clean room environment in which all operations in the clean room have to be shut down until cleanliness can again be achieved at a cost of thousands of dollars an hour. For instance, if the heating element must be replaced in the hot plate, not only must the system be shutdown for the replacement, but following the replacement of the heating element the system will have to be recalibrated prior to restarting the system and the cleanliness procedure followed to reestablish cleanliness in the clean room.
In the operation of the heating or cooling assemblies, the wafers are placed either directly upon the heating/cooling surface of the plate, or, alternatively, upon a plurality of receiving pins, from which the wafers are placed on the surface using an assembly such as that described in U.S. Pat. No. 4,955,590 issued to Narushima et al. The use of receiving pins and/or a table that reciprocates is a preferred method of loading in the industry because it provides access to the exposed uncoated surface for the loading and unloading of the wafers with automated handling equipment when the wafer is seated upon the receiving pins. One problem with this method as discussed in the Narushima patent (col. 1, lines 38-41) is that, if the receiving pins are lowered, the air resistance causes the wafer to float, which can result in misalignment of the wafer on the pins. The Narushima patent (col. 4, lines 37-44) indicates that by moving the table and not the pins this problem is eliminated, because the wafer is not moved; however, the raising of the table will exert a force on the bottom of the wafer that is analogous to the force exerted when the wafer is lowered, thus floating of the wafer will occur even when the table is raised and the pins are stationery. A possible solution to this problem suggested in the Narushima patent (col. 5, lines 3-6) is to draw a vacuum through the distal end of the receiving pins to chuck the wafer against the distal end of the pins to prevent movement. While this solution appears to provide a more plausible method of preventing the wafer from floating, the method greatly complicates the overall design of the system. This is because the wafer must be removed from the receiving pins requiring that the vacuum be released when the wafer reaches the table either through the use of a sensing system or by moving the table at a speed so as to dislodge the wafer from the receiving pins; however, this type of mechanical release would most likely result in misalignment problems and could also potentially damage the wafer. As such, there is need for an improved apparatus and method for receiving wafers, and plate-like material in general, in a plate-like material treating apparatus.
A number of methods exist in the prior art to hold the wafer in position on the surface of the plate following the transfer of the wafer from the receiving pins to the plate. One method is to directly place the wafer on the plate surface and to apply a vacuum through a hole in the surface adjacent to the wafer to hold the wafer in place, as discussed in the Narushima patent (col. 2, lines 50-55). A problem with this method is that uneven heating or cooling of the wafer occurs especially in the vicinity of the holes provided for the receiving pins and for the applying the vacuum and due to thermal maldistributions in the remainder of the plate. An alternative to directly placing the wafer on the surface has been to use ball shaped supports that are press fit into the top surface of the plate thereby creating an air layer between the wafer and the surface that would tend to more uniformly distribute the transfer of energy. However, the use of ball shaped supports reintroduces the problem of securing the wafer on the surface. In addition, the air layer between the wafer and surface must be very small (.about.0.1 mm) in order to maintain the desirable heat transfer characteristics associated with the plate heating/cooling technique, thus requiring that very small ball shaped supports be machined and precisely attached the heat transfer surface of the plate. Accordingly, a need exists for an improved apparatus and method for supporting of plate-like material during thermal treating operations.
During the heating of the wafer on the plate, the volatile solvents in the photoresist are evaporated and must be exhausted to prevent condensation in the system and to provide environmental control of the vapors. In the prior art, as shown in FIGS. 1 and 2, as a wafer is heated on a hot plate 3, either by drawing air over the wafer 2 from the annular region 4 and exhausting the vapor through a perforated plate 5 and exhaust port 6 from above the wafer 2 or drawing air through the perforated plate 5 over the wafer from above the wafer and exhausting the vapors from the annular region 4 surrounding the wafer 2. The exhausting of the solvent vapors requires a large throughput of air that must be drawn from outside of the heating assembly resulting in cool air being drawn over the surface of the wafer. The direct contact of the cool air with the surface can produce uneven cooling of the surface resulting in nonuniformities in the photoresist coating. Also, the outside air can introduce contamination directly onto the surface of the photoresist further degrading the coating. In view of the aforementioned, there is a need for an improved exhaust system and more generally a need for an improved thermal conditioning apparatus and method.
The present invention is directed to a self-contained thermal conditioning apparatus and methods of using the same which overcomes, among others, the above-discussed problems so as to provide a more easily controlled and more uniform photoresist coatings for use in semiconductor production.